Mass spectrometers generally include an ion source disposed in a vacuum system for achieving analysis of chemical substances. In the powerful analytical technique known Gas Chromatography-Mass Spectrometry (GC-MS), volatile analytes from mixtures are first separated into individual components in a gas chromatograph (GC) and the separated samples are directly transferred into a mass spectrometer (MS) for subsequent mass analysis. The GC has a tubular column which is heated (or possibly cooled) to a controlled temperature or along a controlled temperature profile in a gas chromatograph oven (GC oven).
For clean separation of analytes, the temperature of a GC column needs to be carefully controlled, often to within a fraction of a degree. Further, in order to increase throughput, the temperature is often not maintained static during an entire separation, but is ramped along a controlled temperature profile. A GC oven for these purposes usually comprises a thermally insulated housing internally accessible through a door, a heating element, and a motor driven fan for stirring the air in the housing. The stirring fan continuously mixes the air within the oven to minimize temperature gradients which could adversely affect the performance of the chemical processes within the GC column. Various baffles or plenums are generally incorporated into the heated compartment of the GC oven in order to direct and control air flow. To facilitate rapid cooling or cool-down, a GC oven often typically comprises intake ports to allow air or gas to bleed into the oven and outlet ports to exhaust hot air or gas from the oven. For use with highly volatile compounds, the temperature of the GC oven may be accurately controlled at low temperatures (slightly above or even below ambient) by feeding air or a cooled gas into the inlet ports.
The effluent from the GC column needs to be transferred from the GC column, to the MS ion source that is held in vacuum. However, during the transfer (performed conventionally by means of a transfer line), it is necessary to maintain a uniform temperature across the length of the transfer line. If a significant temperature gradient exists so that the temperature varies at different points along the transfer line, cold spots may occur to cause condensation from the gas phase of the sample so that it will either not be passed through to the MS or will exhibit excessive chromatographic peak broadening or peak tailing. On the other hand, hot spots that appear may cause some compounds to degrade thermally with a resultant change in their chemical structure. Similar effects can occur even if the transfer line is at a uniform temperature if the temperature of the transfer line is either too cold or too hot during the elution of any given chemical compound. Additionally, excessive transfer line temperatures can lead to elevated “chemical noise” and lower signal-to-noise ratio for any given analytical results.
Prior art approaches for transferring column effluent to a mass spectrometer have employed isothermal, independently heated transfer lines comprising tubing situated between a gas chromatograph and a mass spectrometer and through which the GC column is passed. As one example, FIG. 1A illustrates a first conventional system for interfacing a gas chromatograph 10 to a mass spectrometer 20. The gas chromatograph 10 comprises a gas chromatograph oven having an insulated oven housing 19. The oven has a temperature controlled oven interior volume 18 containing at least a portion of GC column 12. The mass spectrometer 20 comprises housing 29 that has an interior 28 containing ion source 22. The mass spectrometer interior 28 is generally under vacuum during operation of the mass spectrometer. A portion of the GC column 12 passes through the full length of the interior of a transfer tube 14 and into the ion source 22. The GC column 12 is sealed to the transfer tube by vacuum fitting 13 and the transfer tube 14 is sealed to the mass spectrometer 20 by seal 16. As in other conventional systems for interfacing a gas chromatograph to a mass spectrometer, a portion of the GC column 12 resides within a section of the transfer tube 14 that is neither within the GC oven interior 18 nor the MS interior 28. The conventional system shown in FIG. lA maintains this section at an appropriate temperature by means of a heating tape 11 wrapped around and in close thermal contact with the transfer tube 14. Resistance heating produced by electrical current supplied by electrical leads 15 elevates the temperature of the heating tape 11 and, consequently, of the sections of the transfer tube in contact with the heating tape and the GC column within the transfer tube.
FIG. 1B illustrates a second conventional system for interfacing a gas chromatograph 10 to a mass spectrometer 20. In the system shown in FIG. 1B, a separate box-like oven 17 that encloses a portion of the transfer tube is used instead of heating tape. Power is supplied to the oven 17 by electrical leads 15.
FIG. 1C illustrates a third conventional system for interfacing a gas chromatograph 10 to a mass spectrometer 20. The system shown in FIG. 1C comprises a transfer line 30 disposed between the gas chromatograph 10 and the mass spectrometer 20 that includes two additional tubes—a middle tube 32 and an outer tube 33—that enclose the transfer tube 14, which comprises an inner tube. The middle tube encloses, in addition to the transfer tube, a temperature sensor (not shown) and a heater (not shown) that extends along the full length of the middle tube adjacent to the inner tube. The space between the middle tube 32 and the outer tube 33 acts as insulation, thereby limiting heat transfer to the outer tube. This space may be under vacuum in order to provide thermal insulation, or may be packed with an insulative material such as glass or ceramic fibers.
These conventional approaches have experienced problems of either complexity, increased difficulty of accessing the GC column, non-uniformity of heat distribution within the transfer line, or non-matching of the transfer line temperature to the internal temperature of the GC oven. Although it would be possible to controllably ramp the interface temperature in accordance with the GC oven profile, the thermal mass of such devices precludes convenient and rapid cooldown to the initial conditions necessary for subsequent analysis. Further, using these conventional approaches, it is difficult to maintain a controlled temperature of the transfer line at near ambient conditions or at sub-ambient conditions.